Transparent electrodes are essential parts for many optoelectronic devices such as touch screens, liquid crystal display, and solar cells. Currently, commercial transparent electrodes are made from indium doped tin oxide. The cost of indium and the brittle nature of ITO drive the search for alternative transparent electrode materials. During the past decade, many alternative transparent electrode materials were studied, for instance: conducting polymers, graphene, carbon nanotube (CNT), metal nanowire, and hybrid materials. Those materials can be categorized into two forms: the continuous transparent conductive film (graphene, ITO and conducting polymer) and transparent percolation-conductive film (CNT, metal nanowire and hybrid materials).
In continuous transparent conductive materials, graphene has the highest carrier mobility and optical transparency. Such performance is only available on small, nonscalable mechanically cleaved graphene. The large graphene samples from other sources, such as chemical vapor deposited graphene and solution-processed, reduced graphene oxide, cannot provide comparable perfection (carrier mobility) as small cleaved graphene. The carrier mobility in large area graphene samples is limited by the production techniques, so doped graphene was developed by several groups to enhance the conductivity by increasing the carrier density. Some doped graphene based transparent electrodes exhibit performance similar to ITO. The doped graphene sheets are less stable than the pristine graphene and the fabrication cost are likely to be larger. The high processing cost (such as chemical vapor deposition) and tedious transferring processes hinder its use in transparent electrodes. As a result, ITO is presently the best choice for continuous transparent electrode materials due to its combined properties of performance, cost, stability, and toughness.
The transparent percolation-conductive film is another emerging material for making high performance transparent electrodes. In contrast to continuous transparent conductive film, the transparent percolation conductive film can be made from opaque materials such as carbon nanotubes (CNTs) and metal nanowires. Carbon nanotubes are used to make transparent electrodes because of their extremely high conductivity and aspect ratio. The transparent electrodes made of CNTs are poorer than ITO due to the presence of semiconductive single walled carbon nanotubes and high contact resistance. Metal nanowire transparent electrodes have emerged as better transparent electrodes than CNT electrodes. The best solution-processed copper nanowire and silver nanowire based electrodes exhibit better performance than ITO. However, the solution-processed metal nanowires often contain organic residues that result in lower conductivity in comparison with that of patterned metal nanowire made from evaporated metal. Recent research also indicates that solution-processed metal nanowire transparent electrodes may have poor adhesion and protrusions that limit their use in many devices. Patterned metal nanowires from evaporated metal sources exhibit the best transparent electrode performance. However, a high throughput patterning method to generate the high aspect ratio nanowires are crucial for this type of transparent electrode.
Electrospinning is a facile and economical way to produce continuous nanofiber structures. Previously, it was used to generate continuous conductive nanofibers for transparent electrodes. The directly electrospun conductive nanofibers are usually polymer metal composites. A heat treatment annealing process is required to eliminate the organic residue and obtain reasonable conductivity. This process is not only energy and time-consuming but also limited to substrates that are stable at high temperatures. Cui's group coated metal on continuous electrospun fibers. Wu, H.; Kong, D.; Ruan, Z.; Hsu, P. C.; Wang, S.; Yu, Z.; Carney, T. J.; Hu, L.; Fan, S.; Cui, Y. A Transparent Electrode Based on a Metal Nanotrough Network. Nat. Nanotechnol. 2013, 8, 421-425. The small diameter and ultralong fiber lead to a metal nanotrough network electrode with remarkable performance (sheet resistance 2Ω/□ at 90% transmittance). However, the metal nanotrough network needs to be transferred onto the target substrate, which generates defects and requires additional care to ensure adhesion. It has also been demonstrated that a patterned metal mesh with graphene can provide excellent transparent electrode (sheet resistance 20Ω/□ at 91% transmittance). Zhu, Y.; Sun, Z.; Yan, Z.; Jin, Z.; Tour, J. M. Rational Design of Hybrid Graphene Films for High-Performance Transparent Electrodes. ACS Nano 2011, 5, 6472-6479. This method involves expensive photolithography.
In light of the current state of the art, there is a need in the art for methods to fabricate transparent electrodes with high performance, low cost, and high throughput that can replace ITO.